As feature sizes in integrated circuits approach 0.25 .mu.m and below, problems with interconnect RC delay, power consumption and signal cross-talk have become increasingly difficult to resolve. It is believed that the integration of low dielectric constant materials for interlevel dielectric (ILD) and intermetal dielectric (IMD) applications will help to solve these problems. While there have been previous efforts to apply low dielectric constant materials to integrated circuits, there remains a longstanding need in the art for further improvements in processing methods and in the optimization of both the dielectric and mechanical properties of such materials used in the manufacture of integrated circuits.
Nanoporous Films
One type of material with a low dielectric constant are nanoporous films prepared from silica, i.e., silicon-based materials. Air has a dielectric constant of 1, and when air is introduced into a suitable silica material having a nanometer-scale pore structure, such films can be prepared with relatively low dielectric constants ("k"). Nanoporous silica materials are attractive because similar precursors, including organic-substituted silanes, e.g., tetraethoxysilane ("TEOS"), are used for the currently employed spin-on-glasses ("S.O.G.") and chemical vapor disposition ("CVD") of silica SiO.sub.2. Nanoporous silica materials are also attractive because it is possible to control the pore size, and hence the density, material strength and dielectric constant of the resulting film material. In addition to a low k, nanoporous films offer other advantages including: 1) thermal stability to 900.degree. C., 2) substantially small pore size, i e at least an order of magnitude smaller in scale than the microelectronic features of the integrated circuit), 3) as noted above, preparation from materials such as silica and TEOS that are widely used in semiconductors, 4) the ability to "tune" the dielectric constant of nanoporous silica over a wide range, and 5) deposition of a nanoporous film can be achieved using tools similar to those employed for conventional S.O.G. processing.
Thus, high porosity in silica materials leads to a lower dielectric constant than would otherwise be available from the same materials in nonporous form. An additional advantage, is that additional compositions and processes may be employed to produce nanoporous films while varying the relative density of the material. Other materials requirements include the need to have all pores substantially smaller than circuit feature sizes, the need to manage the strength decrease associated with porosity, and the role of surface chemistry on dielectric constant and environmental stability.
Density (or the inverse, porosity) is the key parameter of nanoporous films that controls the dielectric constant of the material, and this property is readily varied over a continuous spectrum from the extremes of an air gap at a porosity of 100% to a dense silica with a porosity of 0%. As density increases, dielectric constant and mechanical strength increase but the degree of porosity decreases, and vice versa. This suggests that the density range of nanoporous films must be optimally balanced to achieve the desired range of low dielectric constant, and the mechanical properties acceptable for the desired application.
Nanoporous silica films have previously been fabricated by a number of methods. For example, nanoporous films have been prepared using a mixture of a solvent and a silica precursor, which is deposited on a substrate suitable for the purpose.
When forming such nanoporous films, em., by spin-coating, the film coating is typically catalyzed with an acid or base catalyst and additional water to cause polymerization/gelation ("aging") and to yield sufficient strength so that the film does not shrink significantly during drying.
Another previously proposed method for providing nanoporous silica films was based on the observation that film thickness and density/dielectric constant can be independently controlled by using a mixture of two solvents with dramatically different volatility. The more volatile solvent evaporates during and immediately after precursor deposition. The silica precursor, typically partially hydrolyzed and condensed oligomers of TEOS, is applied to a suitable substrate and polymerized by chemical and/or thermal means until it forms a gel. The second solvent, called the Pore Control Solvent (PCS) is usually then removed by increasing the temperature until the film is dry. Assuming that no shrinkage occurs after gelation, the density/dielectric constant of the final film is fixed by the volume ratio of low volatility solvent to silica, as described by EP patent application EP 0 775 669 A2. EP 0 775 669 A2 shows a method for producing a nanoporous silica film by solvent evaporation of one or more polyol solvents, e.g., glycerol, from a metal-based aerogel precursor, but nevertheless fails to provide a nanoporous silica film having sufficiently optimized mechanical and dielectric properties, together with a relatively even distribution of material density throughout the thickness of the film.
Yet another method for producing nanoporous dielectrics is through the use of sol-gel techniques whereby a sol, which is a colloidal suspension of solid particles in a liquid, transforms into a gel due to growth and interconnection of the solid particles. One theory that has been advanced is that through continued reactions within the sol, one or more molecules within the sol may eventually reach macroscopic dimensions so that they form a solid network which extends substantially throughout the sol. At this point, called the gel point, the substance is said to be a gel. By this definition, a gel is a substance that contains a continuous solid skeleton enclosing a continuous liquid phase. As the skeleton is porous, the term "gel" as used herein means an open-pored solid structure enclosing a pore fluid. Removal of the pore fluid leaves behind empty pores.
Protecting the Surfaces of Nanometer Scale Pores
As the artisan will appreciate, a useful nanoporous film must meet a number of criteria, including having a dielectric constant ("k") falling within the required value range, having a suitable thickness ("t") (e.g., measured in .ANG.ngstroms), having an ability to effectively fill gaps, such as, e.g., interconductor and/or intercomponent spaces, on patterned wafers, and having an effective degree of hydrophobicity. If the film is not strong enough, despite achieving the other requirements, the pore structure may collapse, resulting in high material density and therefore an undesirably high dielectric constant. In addition, the surfaces of the resulting nano-scale pores carry silanol functional groups or moieties. Silanols, and the water that can be adsorbed onto the silanols from the environment, are highly polarizable and will raise the dielectric constant of the film. Thus, the requirement for hydrophobicity arises from the requirement for a reduced range of dielectric constant relative to previously employed materials. For this reason, preparation of nanoporous dielectric films can also include optional additional processing steps to silylate free silanols on nanopore surfaces of the film, with a capping reagent, e.g., trimethylsilyl [TMS, (CH.sub.3).sub.3 SiO--] or other suitable, art-known hydrophobic reagents.
Therefore, despite the availability of previous methods for preparing nanoporous silica films, the art recognizes a need for further, ongoing improvements in both nanoporous silica films and methods for preparing the same. In particular, there remains a continuing need in the art for new processes which eliminate some or all of the aforementioned problems, such as providing methods for making silica nanoporous films of sufficient mechanical strength that are also optimized to have a desirably low and stable dielectric constant, without the need for further processing to make the film hydrophobic.